Solar cell with epitaxially grown quantum dot material

ABSTRACT

A monolithic semiconductor photovoltaic solar cell comprising a plurality of subcells disposed in series on an electrically conductive substrate. At least one subcell of the plurality of subcells includes an epitaxially grown self-assembled quantum dot material. The subcells are electrically connected via tunnel junctions. Each of the subcells has an effective bandgap energy. The subcells are disposed in order of increasing effective bangap energy, with the subcell having the lowest effective bandgap energy being closest to the substrate. In certain cases, each subcell is designed to absorb a substantially same amount of solar photons.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/038,230, filed Jan. 21, 2005. U.S. patent application Ser. No.11/038,230 claims the benefit of U.S. Provisional Application No.60/537,259, filed Jan. 20, 2004. U.S. patent application Ser. No.11/038,230 and U.S. Provisional Application No. 60/537,259, areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to photovoltaic solar cells.More particularly, the invention relates to III-V multijunctionsemiconductor solar cells having an epitaxially grown quantum dotmaterial.

BACKGROUND OF THE INVENTION

The sun emits a wide optical spectrum that peaks in the visible and has60% of its photon flux in the wavelength range spanning from ˜350 nm to˜1350 nm. This wavelength range corresponds to 80% of the sun's totalpower flux of ˜1.3 kW/m² at the earth.

It has been known for decades that the best approach to convert thesun's optical power into electrical power is through solar cells thatmake use of absorption transitions in semiconductors. Photon energy isharnessed in this way by exciting electrons from the semiconductor'svalence band across the bandgap into the conduction band. Thephotocarriers thus generated, i.e. the electrons and holes, are thenswept across a p-n or p-i-n junction fabricated by doping differentregions of the semiconductor structure, and are used to produceelectricity. Semiconductors or semiconductor alloys with bandgaps E_(g)absorb impinging photons having energies greater than or equal to E_(g)as opposed to photons having energies less than E_(g). Equivalently, itcan be said that photons having wavelengths corresponding to energiesgreater than E_(g) are absorbed while photons having longer wavelengthsare not.

Since the energy of a photon in excess of E_(g) is effectively lostthrough thermal processes, it is well established that a combination ofmaterials having different bandgaps must be used to adjust the voltageand current of the solar cell in order to optimize the conversionefficiency of solar light into electricity. To that effect,multijunction solar cells, also known as tandem solar cells, have beendeveloped for applications requiring higher conversion efficiencies.

From a fabrication and crystal perspective, the choice of semiconductorsor semiconductor alloys is practically restricted to materials that canbe grown on common substrates, such as GaAs, Ge, Si, or InP substrates,with a minimum of defects. To date, the best optical to electricalconversion efficiencies, the conversion efficiency being defined as theelectrical power that the device can produce divided by the opticalpower received from a light source such as, for example, the sun, arearound 30% and have been achieved by growing a monolithic multijunctioncell having a GaInP top subcell (E_(g)˜1.8 eV), a GaAs middle subcell(E_(g)˜1.4 eV), and a Ge bottom subcell (E_(g)˜0.7 eV) on a Gesubstrate. Since the subcells are typically connected in series throughtunnel junctions, it is recognized that to improve further theconversion efficiency, the bandgaps of the materials have to be changedor, a fourth subcell added.

The total voltage of the multijunction cell is essentially the sum ofthe voltages generated by the individual subcells, where the voltage ofeach subcell is proportional to the subcell's bandgap. To optimize theconversion efficiency, the subcells should be current-matched, otherwisethe subcell generating the weakest current limits the overall current.In the case above, GaInP has a bandgap that can absorb 25% of the totalsolar photon flux (sometimes referred to as the AM0 spectrum), whereasonly 14% of the total solar photon flux transmitted through the GaInPsubcell can be absorbed by the GaAs subcell, and 38% of the total solarphoton flux transmitted through the GaAs subcell can be absorbed by theGe subcell.

This clearly leads to a current imbalance in the multijunction cell.Relatively speaking, the GaAs subcell does not absorb enough solarphotons while the Ge subcell captures too many. To equilibrate thecurrent balance between the subcells, the middle subcell, i.e. thesubcell disposed between the GaInP and the Ge subcells, would have asmaller bandgap. For example, a middle subcell having a bandgap of ˜1.16eV (corresponding to an optical wavelength of approximately 1100 nm)would imply that all the three subcells would each absorb approximately25% of the total solar photon flux. The remaining 25% of the solarphoton flux would not be absorbed since the three subcells aretransparent to the longer wavelength photons (i.e. photons withwavelengths greater than 1.8 μm are not absorbed).

As mentioned above a four-subcell arrangement can improve the currentbalance. If a material with Eg˜1.0 eV is introduced between the GaAs andthe Ge subcells, it yields the following distribution in the absorptionof the solar photon flux: ˜25% of the photons absorbed by the firstsubcell (GaInP), ˜14% by second subcell (GaAs), ˜19% by the thirdsubcell (Eg˜1 eV), and ˜19% by the fourth subcell (Ge). However, thisfour-subcell arrangement is still current-limited by the GaAs subcell.To make the four-subcell arrangement better balanced in terms ofcurrent, the thickness of the first subcell can be adjusted (reduced) tolet some of the shorter wavelength photons reach the second subcell. Inthis scenario, the second subcell absorbs more photons having energiesgreater than that of its bandgap thus leading to more thermally wastedenergy. This was described by Olson et al. in U.S. Pat. No. 5,223,043incorporated herein by reference.

Research and development to find new materials and novel multijunctionarrangements to improve the efficiency of solar cells has been veryactive. For example, Olson, in U.S. Pat. No. 4,667,059, disclosed dualGaInP/GaAs cells on a GaAs substrate; Ho et al., in U.S. Pat. No.5,405,453, disclosed dual GaInP/GaAs cells on a Ge substrate; Wanlass,in U.S. Pat. No. 5,019,177, disclosed dual InP/GaInAsP cells on InP;Freundlich et al., in U.S. Pat. No. 5,407,491, disclosed dual InP/InGaAscells on an InP substrate; Chang et al., in U.S. Pat. No. 5,330,585,disclosed the dual AlGaAs/GaAs cells on a GaAs substrate; these patentsbeing incorporated herein by reference.

These examples of dual cells and the triple cell made of GaInP/GaAs/Geon a Ge substrate can have conversion efficiencies close to 30% as longas compromises in the design or in the quality of the materials aremade. The compromise in the case of the dual cells having GaAs as thesmallest bandgap is that the longer wavelength photons are not absorbed,they are transmitted though all the layers. In the case of dual cellswith the smaller InGaAs or InGaAsP bandgaps, the compromise is that theshorter wavelength photons are losing their excess energies in heat. Itis also worth nothing that GaAs or Ge substrates have the advantage of alower cost compared to InP substrates.

To extend the photo-absorption of GaInP/GaAs cells to longerwavelengths, Freundlich, in U.S. Pat. No. 6,372,980, incorporated hereinby reference, disclosed solar cells with InGaAs quantum wells, the solarcells having modeled efficiencies in excess of 30%. Other schemes havealso been disclosed to try to improve the efficiency of solar cells. Forexample, Freundlich et al., U.S. Pat. No. 5,851,310, incorporated hereinby reference, disclosed the use of strained quantum wells grown on anInP substrate. Also, Suzuki in U.S. Pat. No. 6,566,595 (later referredto as '595), incorporated herein by reference, disclosed the use ofquantum well layers having a plurality of projections with differentsizes, the goal being of better matching the sun's spectrum by usingmaterials having different bandgaps.

Similar is the disclosure by Sabnis et al. in U.S. Pat. No. 4,206,002,incorporated herein by reference, for bulk graded bandgap multijunctionsolar cells. In the case of the '595 patent however, the overallefficiency is unlikely to be improved since the tailoring of theabsorption spectrum involves distributing quantum well or quantum dotmaterials of different sizes in the plane of the layers. Thiscompromises the spatial density of the material that can be used toabsorb light and is likely to require thicker layers to absorb the samenumber of photons as would be absorbed in uniform layers or, largersurfaces which would reduce the conversion efficiency. Chaffin et al.,in U.S. Pat. No. 4,688,068, incorporated herein by reference, alsodisclosed the use of quantum wells in multijunction cells.

As disclosed by Kurtz et al. in U.S. Pat. No. 6,252,287, incorporatedherein by reference, InGaAsN lattice-matched to GaAs is also a promisingmaterial for tailoring the bandgap of layers lattice-matched to GaAs foroptimizing the conversion efficiency.

Other aspects of the fabrication of monolithic multijunction solar cellssuch as antireflection windows, tunnel junctions, and surfacemetallization have matured with the extensive developments ofphotovoltaic solar cells as disclosed in numerous patents andpublication in that field as seen in several of the patents identifiedabove (for example, U.S. Pat. Nos. 4,694,115; 5,009,719; 4,419,530;4,575,577).

In the field of semiconductor nanostructures, it is well known thathigh-quality, defect-free, self-assembled quantum dots can be obtainedduring the early stage of growth of highly strained semiconductors (seefor example: S. Fafard, et al., “Manipulating the Energy Levels ofSemiconductor Quantum Dots”, Phys. Rev. B 59, 15368 (1999) and S.Fafard, et al., “Lasing in Quantum Dot Ensembles with Sharp AdjustableElectronic Shells”, Appl. Phys. Lett. 75, 986 (1999)). Such quantum dotmaterial can be grown in multiple layers to achieve thick active regionsfor devices such as Quantum Dot Infrared Photodetectors, as disclosed byFafard et al. in U.S. Pat. No. 6,239,449, incorporated herein byreference. There, the interband absorption properties of the quantum dotmaterial can be tailored to cover various wavelength ranges in the nearinfrared and visible portions of the optical spectrum. The composition,size and shape of the quantum dot material are adapted to change thequantization energies and the effective bandgap of the quantum dotmaterial, where the effective bandgap of the material is defined asessentially being the lowest energy transitions at which photons can beabsorbed and is determined by the quantized energy levels of theheterostructure.

Self-assembled quantum dots come in a wide range of high qualitymaterials that can be pseudomorphically grown on GaAs or InP. Forexample, InAlAs/AlGaAs on GaAs substrates absorbs in the red or thenear-infrared, InAs/InAlAs on InP substrates absorbs in the 1.5 μmwavelength range, and InAs/InGaAs on InP substrates absorbs in the 1.9μm wavelength range. More importantly however, In(Ga)As/GaAsself-assembled quantum dot material grown on GaAs substrates, isparticularly well-suited for absorption below the GaAs bandgap in thespectral region spanning from 885 nm to 1150 nm, or up to ˜1350 nmdepending on the growth parameters. In(Ga)As/GaAs self-assembled quantumdot layers can be grown uniformly and with high densities. Furthermore,multiple layers can be grown with the same uniformity or, whendesirable, with different sizes and/or compositions by simplycontrolling the growth parameters. Additionally, the In(Ga)As/GaAsself-assembled quantum dot material has been shown to produce deviceswhich are orders of magnitude more radiation robust than conventionalmaterial (see for example: P. G. Piva et al., “Enhanced DegradationResistance of Quantum Dot Lasers to Radiation Damage”, Appl. Phys. Lett.77, 624 (2000)). The radiation and defect hardnesses are particularlygreat advantages for space applications where the solar cells aregetting exposed to radiations.

As can be appreciated from the prior art discussed above, there is areal need for high quality materials having desired absorption spectra,that can be easily incorporated in multijunction solar cells to improvefurther the conversion efficiency. A reliable material that can balancethe absorption between the bandgaps of GaAs and Ge is of particularinterest.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a multijunctionsemiconductor photovoltaic solar cell. The solar cell comprises aplurality of subcells disposed in series, each subcell has formedtherein a p-n junction or a p-i-n junction. The plurality of subcellsincludes a quantum dot subcell, the quantum dot subcell is a subcellwith strained epitaxially-grown semiconductor layers that includeself-assembled quantum dots. The quantum dot subcell ispseudomorphically grown on another subcell.

The solar cell can comprise tunnel junctions formed between thesubcells, the tunnel junctions for electrically connecting the subcells.

The solar cell can comprise an electrically conductive substrate uponwhich the plurality of subcells is formed. Each of the subcells has aneffective bandgap energy, the subcells the subcells can be disposed inorder of increasing effective bandgap energy, a subcell with a lowesteffective bandgap energy being closest to the electrically conductivesubstrate.

The solar cell can be such that each subcell is for absorbing asubstantially same fraction of solar photons.

The plurality of subcells can consist of three subcells, a first subcellhaving the lowest effective bandgap energy, a third subcell having ahighest effective bandgap energy, and a second subcell disposed betweenthe first subcell and the third subcell, the second subcell being thequantum dot subcell. The electrically conductive substrate can be a Geor a GaAs substrate. The first subcell can include Ge. The strainedepitaxially-grown semiconductor layers can include strained InGaAsquantum dot layers intercalated with GaAs, AlGaAs, or GaPAs layers. Thethird subcell can include GaInP, AlGaAs or AlGaInP.

The solar cell of can be such that the first subcell is epitaxiallygrown on the electrically conductive substrate. Alternatively, theelectrically conductive substrate can be a Ge substrate and the firstsubcell can an interdiffused portion of the Ge substrate.

The electrically conductive substrate can be n-doped, and an n-side ofeach of the p-n junction or p-i-n junction formed in each subcell iscloser to the substrate than a respective p-side of the p-n junction orthe p-i-n junction.

The electrically conductive substrate can be p-doped, and a p-side ofthe p-n junction of p-i-n junction formed in each subcell is closer tothe substrate than a respective n-side of the p-n junction or p-i-njunction.

The solar cell can be such that one of the subcells includes a Braggreflector or a distributed Bragg reflector to modify an absorptioncharacteristic of the solar cell.

In a second aspect, the present invention provides a multijunctionsemiconductor photovoltaic solar cell. The solar cell comprises aplurality of subcells disposed in series, each subcell has formedtherein a p-n junction or a p-i-n junction. The plurality of subcellsincludes a quantum dot subcell, the quantum dot subcell is a subcellwith strained epitaxially-grown semiconductor layers that includeself-assembled quantum dots.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexamples only, with reference to the attached Figures, wherein:

FIG. 1 shows the percentage of the AM0 solar photon and power fluxesintegrated from 200 nm.

FIG. 2 depicts a monolithic three-subcell photovoltaic solar cell of thepresent invention.

FIG. 3 shows photovoltaic spectra of self-assembled quantum dotmaterials.

FIG. 4 depicts a self-assembled quantum dot material of the presentinvention.

FIG. 5 depicts a monolithic four-subcell photovoltaic solar cell of thepresent invention.

FIG. 6 depicts a monolithic two-subcell photovoltaic solar cell of thepresent invention.

FIG. 7 depicts a conduction band of a quantum dot.

DETAILED DESCRIPTION

The present invention provides monolithic semiconductor photovoltaicsolar cells comprising at least one subcell having a self-assembledquantum dot material. Also provided is a method for making such solarcells, the method using epitaxial growth of self-assembled quantum dotmaterial in at least one subcell of the solar cell.

The first embodiment is a high efficiency monolithic three-junctionphotovoltaic solar cell. Three-junction photovoltaic solar cells can befabricated by stacking p-n or n-p junctions made of differentsemiconductor materials. As stated above, conversion efficiencies ofapproximately 30% have been obtained using a Ge bottom subcell grown ona Ge substrate, with a GaAs middle subcell and a GaInP or AlGaAs topsubcell. The efficiency of such a multijunction photovoltaic solar cellis improved in the present invention by using self-assembled quantum dotmaterial in the middle subcell instead of bulk GaAs.

The principle of the invention is illustrated in FIG. 1 where plot 10shows the percentage of the AM0 solar photon flux, integrated from the200 nm wavelength, as a function of wavelength, and plot 12 shows thepercentage of the AM0 solar power flux integrated from 200 nm wavelengthas a function of wavelength. Reference numerals on plot 10 indicatesemiconductor materials and their absorption edges. These semiconductormaterials can be used in converting photons (solar or otherwise) toelectrical carriers. The plotted points of FIG. 1 are referred to in thedescription as cells, subcells, particular semiconductor material orabsorption edges, depending on their particular context.

To optimize the conversion efficiency of a solar cell, the current ofeach subcell must be substantially equal since the subcells areconnected in series. Neglecting the reflection at the surface of thedevice, which is a valid approximation for solar cells equipped with anantireflection coating, the light intensity transmitted at a depth zfrom the surface of the semiconductor is given by I(z)=I_(o)exp(−αz).I_(o) is the input intensity and α is the semiconductor absorptioncoefficient, which is a function of the wavelength dependent density ofstates of the material (i.e. α is wavelength dependent). Forsufficiently thick semiconductor material, only light having awavelength longer than the semiconductor bandgap equivalent wavelength(or with an energy less than the bandgap energy) will transmit throughthe semiconductor layer since the density of states drops, as does α,for photon energies less than the bandgap energy. For direct bandgapsemiconductors, at wavelengths shorter than the bandgap wavelength, α isin the 10 ⁴ cm⁻¹ to 10 ⁵ cm⁻¹ range and each impinging photon can createa pair of photocarriers, i.e. an electron and a hole.

The current in a subcell is proportional to the fraction of photon fluxabsorbed by the subcell. As can be inferred from plot 10, aGaInP/GaAs/Ge solar cell absorbs about 25% of the photon flux in its topGaInP subcell 14, approximately 14% in its middle GaAs subcell 16, andapproximately 38% in the bottom Ge subcell 18. Replacing the top GaInPsubcell 14 with an AIGaAs subcell 20 would yield similar results.

The imbalance in the absorption of the solar photon flux by subcells 14,16 and 18 leads to a current imbalance. That is, the Ge bottom subcell18 is generating the most current and the middle GaAs subcell 16 islimiting the overall current and conversion efficiency. The overallconversion efficiency can be improved by substituting GaAs material 16with a material having an effective bandgap wavelength of about 1070 nm(1.16 eV). As will be seen in detail below, such a material can be aself-assembled quantum dot material 22. By using quantum dot material 22in the middle subcell, the solar photon flux absorption of each of thethree subcells is about 25% of the total solar photon flux and thecurrent generated by each subcell will be equal. The theoreticalefficiency can be calculated to give the thermodynamic limit ofphotovoltaic energy conversion. The theoretical efficiency takes intoaccount the bandgap of the subcell, the impinging photon flux and itsspectral distribution to estimate the corresponding open-circuit voltage(V_(oc)) and the short-circuit current (J_(sc)) as described by, forexample, Baur et al in paper 3P-B5-07 of the WCPEC-3 Proceedings Osaka,2003. For an optimized configuration, the conversion efficiency couldtheoretically exceed 40%.

The details of such a monolithic three-junction photovoltaic solar cellis illustrated is FIG. 2 (not to scale), which sketches an embodimentfor the solar cell of the present invention. The multijunction solarcell 24 comprises a substrate 26, first subcell 28, second subcell 30pseudomorphically grown by epitaxy and comprising self-assembledIn(Ga)As/GaAs quantum dot material, and third subcell 32. TheIn(Ga)As/GaAs quantum dot material of second subcell 30 can be tailoredto obtain an effective bandgap of about 1.16 eV. Other techniques can beused to produce similar nanostructures using other epitaxial techniquessuch as selective area epitaxy, templated epitaxy, epitaxy withstained-induced bandgap modified heterostructures, Volmer-Weber growthmodes, modified Stranski-Krastanow growth modes, Frank-Van der Merwegrowth modes combined or not with high-resolution microfabrication, or,non-epitaxial techniques involving, for example, colloidal quantum dots;however, the optical and/or structural properties of such nanostructuresare typically not suited for improving the efficiency of multijunctionsolar cell devices.

According to this embodiment of the invention, substrate 26 can beconductive GaAs or preferably conductive Ge, both of them having asimilar lattice constant. The doping of substrate 26 can be n-type orp-type. Whether substrate 26 is n-type with an n-p or an n-i-p junctiongrown on top or, p-type with a p-n or a p-i-n junction grown on top, isnot fundamental to the present invention. For illustration purposes,this embodiment will use an n-type substrate with n-p or n-i-p junction.Other possible combinations, which could include an undoped substrateand buried back contacts, are equally possible. Substrate 26 may bemetallized to form ohmic contact 34 on the backside, as illustrated inFIG. 2, and a buffer and/or back field layers 36 can be grown betweensubstrate 26 and first subcell 28 to optimize various structural,electrical, or optical properties. First subcell 28 is preferably madeof Ge and includes an n-p junction to create a depletion region. Atunnel junction 38 is used to connect first subcell 28 with secondsubcell 30.

As will be readily understood by a person skilled in the art, tunneljunction 38 is preferably made of a high quality material that can beepitaxially grown on first subcell 28. Tunnel junction 38 is preferablyhighly doped to provide good electrical conduction and to support highcurrent densities and, is preferably transparent to photons traversingit. For this embodiment tunnel junction 38 can be made of a highly dopedGaAs n-p junction but many other combinations supporting therequirements herein are equally valid.

Second subcell 30 comprises a self-assembled In(Ga)As/GaAs quantum dotmaterial, adapted to, or tailored to, obtain an effective bandgap ofabout 1.16 eV. More details about second subcell 30 are given in FIG. 4and its corresponding description where it is disclosed to contain ann-p or an n-i-p junction comprising a plurality of layers withhigh-quality self-assembled In(Ga)As/GaAs quantum dots of a specificshape, composition, and density, grown pseudomorphically by epitaxy.

Second subcell 30 is electrically connected to third subcell 32 viatunnel junction 40. The requirements of tunnel junction 40 are similarto those of tunnel junction 38 discussed above. In this embodimenttunnel junction 40 can be made of a highly doped InGaP or AlGaAs n-pjunction but many other combinations are equally valid. Third subcell 32is essentially an n-p junction preferably made of doped GaInP or dopedAlGaAs, or a similar alloy latticed-matched to GaAs, with a bandgaparound 1.8 eV. Third subcell 32 may include window 42, antireflectionlayer 44, and contact 46 as is customary in multijunction solar cells.

The top part of FIG. 2 shows graph 48 of spectral intensity as afunction of wavelength for solar spectrum 50. FIG. 2 also illustratesabsorption ranges 52, 54 and 56 of solar spectrum 50 for third subcell32, second subcell 30 and first subcell 28 respectively. In view of thediscussion relating to FIG. 1, it will be clear for someone skilled inthe art that such a three-junction photovoltaic solar cell has goodcurrent matching between the subcells together with high conversionefficiencies.

FIG. 3 shows photovoltaic spectrum 58 of high quality self-assembledquantum dot materials, i.e. materials that may be comprised withinsecond subcell 30, grown within a p-i-n junction. Plot 58 shows thespectrum of a first material measured at 20° C. The features of plot 58are quantum dot material ground states 60, quantum dot material excitedstates 62 and wetting layer states 64, a wetting layer being a thincontinuous layer that usually forms during the epitaxy of theself-assembled quantum dots. In this self-assembling epitaxial process,the first monolayer, or first few monolayers, are deposited in uniformtwo-dimensional layers called the wetting layers. Quantum dots thenself-assemble from the additional material deposited and/or in part fromthe previous wetting layer material. Plot 58 was measured using a whitelight source transmitted through GaAs layers. Signal decrease 68 isobserved for energies greater than the GaAs bandgap. Quantum dot groundstates 60 can be referred to as the effective bandgap of aself-assembled quantum dot material. For bulk semiconductors notcomprising semiconductor heterostructures, the effective bandgap issimply the bandgap of the semiconductor material.

It is known that quantum dots energy levels can be adjusted bycontrolling their shape, composition, and density during growth [forexample see: S. Fafard, et al., “Manipulating the Energy Levels ofSemiconductor Quantum Dots”, Phys. Rev. B 59, 15368 (1999) or S. Fafard,et al., “Lasing in Quantum Dot Ensembles with Sharp AdjustableElectronic Shells”, Appl. Phys. Lett. 75, 986 (1999)]. For plot 58, theself-assembled In(Ga)As/GaAs quantum dot material was tailored to havean effective bandgap of about 1.16 eV. The material measured in plot 58contains 14 layers of In(Ga)As quantum dots separated with 10 nmbarriers made of GaAs

An embodiment of second cell 30 is shown in FIG. 4. There, a pluralityof III-V semiconductor alloy layers is grown epitaxially on tunneljunction 38, which comprises heavily p-doped layer 78 and heavilyn-doped layer 80. An emitter 82 is formed by the combination of highlyn-doped layer 84 and n-doped layer 86. Emitter 82 is preferably made ofGaAs or from another alloy lattice-matched to GaAs and has a bandgapclose to the bandgap of GaAs. Similarly, a collector 88 is later grownusing p-type doping for layer 90 and highly p-doped layer 92. Silicon ispreferably used for n-type doping while beryllium is preferably used forp-type doping. Obviously, other dopants may be used such as, forexample, Zinc (Zn), Tellurium (Te), or others.

Emitter 82 and collector 88 form an n-i-p junction together withintrinsically undoped self-assembled quantum dot material 94 disposedbetween emitter 82 and collector 88. The doping profiles of emitter 82and collector 88 are such that they provide a depletion region extendingsubstantially across self-assembled quantum dot material 94. Similarconfigurations can be designed using an n-p junction instead of an n-i-pone, or by reversing the order of the p and n doping. Additionally,since self-assembled quantum dot material 94 is pseudomorphically grownon GaAs, Bragg reflectors or distributed Bragg reflector (DBR) cavitiescomprising alternate layers of high and low index of refractionsemiconductors may be grown within the emitter and/or the collector, toenhance the reflectivity and change the absorption properties of thesubcell and consequently of solar cell 24. Similar self-assembledquantum dot material 94, together with emitter 82 and collector 88, asdepicted in FIG. 4, will be used in the description of all embodimentsof the present invention.

Self-assembled quantum dot material 94 comprises a first quantum dotlayer 96, first barrier 98, second quantum dot layer 100, second barrier102 and so on up to N^(th) self-assembled quantum dot material 104 andN^(th) barrier 106. It will be clear for one skilled in the art that theN quantum dot layers need not be identical in thickness or composition.Furthermore, layers can be inserted in self-assembled quantum dotmaterial 94 to optimize optical, structural or electrical properties ofsolar subcells such as second subcell 30. For example layers with otherbandgaps or with a another lattice constant can be grown above and/orbelow quantum dot layers 96, 100, 104 to modify the optical and/or thestructural properties of quantum dot material 94. Also, layers withother bandgaps or with another lattice constant can be grown withinbarriers 98, 102, 106. The growth of such intermediate layers within thebarriers is particularly important to control the total strain built inthe structure. For example, the thickness of semiconductor layers havinga lattice constant smaller than the epitaxial layer and smaller than thelattice constant of the substrate can be chosen to yield layers thatwould compensate the strain introduce when using a semiconductor with alarger lattice constant for the quantum dot layers. For example, GaPAsor GaInP can be grown within the barriers to compensate the strain ofthe InAs or InGaAs quantum dots. Furthermore, the plurality of layerscould contain a number of sub-groups of layers having similarproperties, such that N layers would be composed of m sub-groups eachcontaining a number m_(i) of quantum dot and barriers layers having asimilar size, composition, and effective bandgap.

In the present embodiment, growth materials and parameters can be chosento obtain self-assembled quantum dot material 94 with desired absorptioncharacteristics such as, for example, an absorption edge at 1.16 eV.Pseudomorphic growth of the self-assembled quantum dot material 94 isobtained by epitaxy using, for example, a molecular beam epitaxy (MBE)system. The MBE system is used for growing, for example, GaAs or AlGaAslayers on a semiconductor material lattice-matched to GaAs. Epitaxysystems other than MBE systems can be used. They may include, forexample, chemical beam epitaxy (CBE), metal organic chemical vapordeposition (MOCVD) or other similar hybrid systems or combinationthereof. To obtain the desired optical, electrical and structuralproperties, the growth temperature is maintained in a range thatoptimizes the desired properties while avoiding high temperatures thatcould cause intermixing of layers or diffusion of dopants present, forexample, in emitter 82 or collector 88 layers.

As an example, when intermixing or diffusion of the dopants in emitter82 layers is not a concern, the growth of the GaAs layers is preferablydone in a temperature range comprised between 400° C. and 800° C.,preferably between 520° C. and 630° C. and most preferably between 600°C. and 630° C. In the case where intermixing and/or diffusion of dopantsis a concern during the epitaxial growth of the quantum dot layers, thegrowth temperature is preferably comprised between of 450° C. and 550°C. and most preferably between of 490° C. and 530° C. The growthtemperature of the quantum dot layers is used to adjust the shape andcomposition of the quantum dots. The temperature during the overgrowthof the barrier of each quantum dot layer may be varied at differentstages of the overgrowth to further control the size and composition ofthe quantum dots and therefore the absorption characteristics ofself-assembled quantum dot material 94.

The combination of growth temperature, the group V over-pressure or theIII/V ratio, the quantum dot material, the amount of material used toobtain the self-assembled growth transition between a uniform quasitwo-dimensional film to three-dimensional islands, the growth rate orthe pauses used during the growth, and the overgrowth conditions such asgrowth temperature and growth rate, are chosen to obtain quantum dotlayers having a high in-plane density of highly uniform quantum dotshaving desired energy levels. This allows high conversion efficienciesof impinging solar photons into electricity.

As will be apparent to one skilled in the art, there are manycombinations of parameters that can accomplish the desired absorptioncharacteristics. However, for illustration purposes of the presentembodiment, the desired absorption of the self-assembled quantum dotmaterial 94 can be obtained by growing InAs on GaAs, the thickness ofInAs being comprised between 0.6 nm and 0.8 nm and preferably comprisedbetween 0.68 nm and 0.72 nm. The preferred growth rate of InAs iscomprised between 0.001 and 3 nm/s and most preferably between 0.01 and0.03 nm/s, with a growth pause following the InAs deposition, the growthpause preferably ranging from 0 to 300 seconds. The growth of the InAsquantum dot layer is followed with the over-growth of a barrier layerhaving a thickness ranging from 6 nm to 50 nm, the barrier layerpreferably being GaAs or A_(x)Ga_(1-x)As, x being comprised between 0and 1 but preferably comprised between 0 and 0.35. The growth sequenceof quantum dot layer and barrier layers is repeated a number of times asstated above.

As mentioned above, a specific temperature cycling of the substrate maybe used to adjust the shape, composition, and uniformity of the quantumdots during the overgrowth of the quantum dot layers and barrier layers.In this case, the temperature of substrate 26 is increased preferablyabove the InAs disorption temperature which is roughly 530° C. for MBEgrowth and can depend, amongst different factors, on the growth methodand on the use of a surfactant. Once the temperature has been increasedabove the InAs disorption temperature, it is decreased back to a nominalvalue preferably comprised between 450° C. and 550° C. and mostpreferably between 490° C. and 530° C. This is followed by the growth ofa subsequent quantum dot layer. In the case where the desired absorptionedge is 1.16 eV, the temperature cycling performed during theover-growth may occur when the thickness of the barrier is comprisedbetween 1 nm and 50 nm, preferably between 2.0 nm and 10.0 nm, and mostpreferably between 4.5 nm and 6.5 nm.

In the preferred embodiment, the number of quantum dot layers iscomprised between 1 and 100 preferably between 30 and 80. It is possibleto grow more layers if necessary. The larger the number of layers ofself-assembled quantum dot layer material 94, the larger the absorptioncoefficient of second cell 30 will be, which is desirable to increasethe current of second cell 30.

The distance between the quantum dot layers, i.e. the barrier thickness,is adjusted to (A) change the desired characteristics of the absorptionspectrum; (B) control the vertical stacking of the self-assembledquantum dots; and (C) maintain the overall strain level below thatrelated to the critical thickness that leads to the onset of latticerelaxation. For thicknesses above the critical thickness, the quantumdot material could start developing material dislocations and defects.The critical thickness can be measured and/or estimated by using, forexample, Matthew's law. For the average InGaAs composition of interestwith low indium content the critical thickness is expected to be between1 and 2 microns. The critical thickness is smaller for higher averageindium content. The distance between the quantum dot layers cantherefore be used to adjust the average indium composition of thequantum dot material and avoid dislocations and defects caused by strainand lattice relaxation. As discussed above, the quantum dot layerstypically have a larger lattice constant than the rest of the structureand therefore the embodiment can also incorporate thin layers ofsemiconductors such as GaPAs or InGaP with the opposite strain (i.e.smaller lattice constant) for the purpose of reducing the average strainin the quantum dot material if necessary. For example, as mentionedabove, layers with a different lattice constant can be grown aboveand/or below the quantum dot layers 96, 100, 104 to modify thestructural properties of the quantum dot material 94, or similarlylayers with a different lattice constant can be grown within thebarriers 98, 102, 106.

A second embodiment of the invention provides high efficiency monolithicfour-junction photovoltaic solar cells. Self-assembled quantum dotmaterial 94 may be adapted to absorb photons with energies greater thanabout 1.0 eV, indicated as material 21 in FIG. 1. Such a material can beused to fabricate a high-efficiency monolithic four-junctionphotovoltaic solar cell depicted in FIG. 5 where the subcells will becurrent-matched if each of the subcells absorbs about 19% of the solarflux.

The four-junction solar cell comprises substrate 108 upon which firstsubcell 110 is fabricated. First subcell 110 preferably includesgermanium with an appropriate doping profile, the germanium being grownon substrate 108 by epitaxy or by other crystal growth methods.Alternatively, first subcell 110 can be fabricated by intermixing orimplanting dopants in bulk germanium material such as, for example, a Gesubstrate to create the appropriate doping profile. For example, whenIII-V semiconductor materials are grown on p-type Ge, the intermixing ofthe group V within the Ge of the substrate will form an n-type Ge regionand therefore a p-n junction. Similarly for an n-type Ge substrate, theintermixing of the group III within the Ge of the substrate would form ap-type region and therefore an n-p junction. Second subcell 112 ispseudomorphically grown on first subcell 110 by epitaxy and comprises aself-assembled In(Ga)As/GaAs quantum dot material adapted to obtain aneffective bandgap of about 1.0 eV. Third subcell 114 ispseudomorphically grown on second subcell 112 by epitaxy and is followedby fourth subcell 116, which is pseudomorphically grown on third subcell114 by epitaxy.

In this embodiment, substrate 108 can be conductive GaAs or preferablyGe, each of them having a similar lattice constant. The doping ofsubstrate 108 can be n-type or p-type. Whether substrate 108 is n-typewith an n-p or an n-i-p junction grown on top or, p-type with a p-n or ap-i-n junction grown on top, is not fundamental to the presentinvention. For illustration purposes, this embodiment will use an n-typesubstrate with n-p or n-i-p junctions. Other possible combinations,which could include an undoped substrate and buried back contacts, areequally possible. In the final steps of the process, substrate 108 maybe metallized to form ohmic contact 118, as illustrated in FIG. 5.

A buffer and/or back field layer 120 can be fabricated on substrate 108prior to the growth of first subcell 110 to optimize various structural,electrical, or optical properties. First subcell 110 is preferably madeof Ge and includes an n-p junction to provide a depletion region. Tunneljunction 122 is used to connect first subcell 110 with second subcell112. As will be readily understood by a person skilled in the art,tunnel junction 122 is preferably made of a high quality material whichcan be epitaxially grown on first subcell 110 and is highly doped toprovide good electrical conduction and to support high currentdensities. Tunnel junction 122, as all tunnel junctions describedherein, is preferably substantially transparent to photons traversingit.

For this embodiment the tunnel junction 122 can be made of a highlydoped GaAs n-p junction but other types of tunnel junctions are possiblesuch as AlGaAs, or alloys of AlGaInAsP with a lattice constant close tothat of GaAs. As previously stated, second subcell 112 comprisesself-assembled In(Ga)As/GaAs quantum dot material tailored to obtain aneffective bandgap of about 1.0 eV. The details of second subcell 122 aresimilar to the ones disclosed in FIG. 4 and its correspondingdescription, but with modifications as far as the growth parameters ofthe self-assembled quantum dot material. Further details regarding thegrowth of second subcell 112 appear below. For now, suffice to say thatsecond subcell 112 comprises a self-assembled quantum dot material andan n-p or n-i-p junction. The self-assembled quantum dot materialincludes a plurality of layers with high quality self-assembledIn(Ga)As/GaAs quantum dots of specified shape, composition, and densitygrown pseudomorphically by epitaxy.

Second subcell 112 is connected to third subcell 114 via tunnel junction124. Tunnel junction 124 is preferably made of a high quality material,which can be epitaxially grown on third cell 112 and is highly doped toprovide good electrical conduction and to support high currentdensities. Tunnel junction 124 is substantially transparent to photonstraversing it. In this embodiment, tunnel junction 124 can be made of ahighly doped GaAs, InGaP, AlGaAs, or AlGaInAsP n-p junction, the alloyused having a lattice constant close to that of GaAs and a bandgap equalor greater than that of GaAs.

Third subcell 114 is essentially an n-p junction preferably made ofdoped GaAs or of an AlGaInAsP or GaInNAs alloy latticed-matched to GaAsand having a bandgap around 1.4 eV. For some configurations, to helpbalance the current of the subcells, it might be desirable to adjust thethickness and absorption characteristics of third subcell 114 such thatthird subcell 114 lets part of the light impinging on it reach secondsubcell 112. Third subcell 114 is connected to fourth subcell 116 viatunnel junction 126 which can be made of a highly doped InGaP or AlGaAsn-p junction but other alloys supporting the requirements mentionedabove are equally valid, for example AlInGaP or ZnSe alloys.

Fourth subcell 116 is essentially an n-p junction preferably made ofdoped GaInP or AlGaAs, or a similar AlGaInAsP alloy latticed-matched toGaAs, and has a bandgap around 1.8 eV. Preferably, the thickness andabsorption characteristics of fourth subcell 116 are such that fourthsubcell 116 lets part of the light impinging on it reach third subcell114. Furthermore, fourth subcell 116, third subcell 114, second subcell112 and first subcell 110 are such that the respective currentsgenerated by photons absorbed by the respective cells are balanced.Fourth subcell 116 may include window 128, antireflection coating 130,and electrical contact 132, as is customary in the fabrication ofmultijunction solar cells.

The top part of FIG. 5 shows graph 134 of the spectral intensity as afunction of wavelength for solar spectrum 50. FIG. 5 also illustratesabsorption ranges 136, 138, 140 and 142 of solar spectrum 50 for fourthsubcell, 116, third subcell 114, second subcell 112 and first subcell110 respectively. It will be clear for someone skilled in the art, withthe help of the description of FIG. 1, that such a four-junctionphotovoltage solar cell will have good current matching between thesubcells with about 19% of the solar photon flux absorbed in eachsubcell, and consequently, high conversion efficiencies.

A measured photovoltaic spectrum of a high quality self-assembledquantum dot material grown within a p-i-n junction and having anabsorption band edge of about 1.0 eV is shown as plot 70 in FIG. 3.There, plot 70, measured at 20° C., shows spectral features associatedwith the quantum dot ground states 71, the quantum dot excited states 72and the wetting layer states 76. This particular sample contains onesingle layer of In(Ga)As quantum dots embedded in GaAs barriers.

The growth conditions of second subcell 112 can be adjusted so that thesize and the composition of the quantum dots, together with thecomposition of the material adjacent to the quantum dots, yield aself-assembled quantum dot material with an absorption edge at an energylower than 1.16 eV at about 1.0 eV. As discussed previously, there aremany combinations of growth parameters that can accomplish the desiredgoal. However, for illustration purposes of the embodiment justdescribed, the desired absorption characteristics of self-assembledquantum dot material 94 can be obtained by growing InAs on GaAs, thethickness of InAs being comprised preferably between 0.4 nm and 0.8 nm,and most preferably between 0.50 nm and 0.58 nm. The preferred growthrate of InAs is comprised between 0.001 and 3 nm/s and more preferablybetween 0.01 and 0.03 nm/s, with a growth pause following the InAsgrowth, the growth pause preferably ranging from 0 second to 300seconds. The growth of the InAs quantum dot layer is followed by theover-growth of a barrier layer having a thickness ranging from 6 nm to50 nm, the barrier layer preferably being a GaAs or an AlGaAs alloy withAl composition smaller than about 10% having a slightly higher bandgap.The growth sequence of quantum dot layer and barrier layers is repeateda number of times as previously stated. The preferred number of quantumdot layers is between 50 and 150 layers, or as required to balance theabsorption in the subcells.

As mentioned above, a specific temperature cycling of substrate 108 maybe used to adjust the shape, composition, and uniformity of the quantumdots during the overgrowth of the quantum dot layer. In this case, thetemperature of substrate 108 is increased preferably above the InAsdisorption temperature and decreased back to its nominal value beforethe growth of the subsequent quantum dot layer. In this case where thedesired absorption edge is about 1.0 eV, the temperature cyclingperformed during the over-growth may occur when the thickness of thebarrier is comprised between 1 nm and 50 nm, and preferably between 2.0nm and 10.0 nm, and most preferably between 7.5 nm and 10.0 nm.Alternatively, alloys of slightly lower bandgap material than GaAs suchInGaAs with low concentration of indium, or graded bandgap materials,can also be used adjacent to the quantum dot layers to extend absorptionto longer wavelengths.

A method for incorporating self-assembled quantum dot material within asubcell of a multijunction monolithic photovoltaic solar cell maycomprise the steps of: providing a substrate having a top surface with alattice constant; providing a subcell, lattice-matched to said latticeconstant, upon the previous layer; providing a tunnel junction,lattice-matched to said lattice constant, upon the previous subcell andrepeating the last two steps until the subcell comprising self-assembledquantum dot material has to be incorporated. The last two steps areomitted if the self-assembled quantum dot material has to beincorporated in the bottom subcell. Further steps include: epitaxiallydepositing buffer layers, upon the previous layer, of semiconductormaterials that are lattice-matched to the said lattice constant andhaving a buffer dopant concentration; epitaxially depositing back fieldlayers, upon said buffer layers, of a semiconductor materials that arelattice-matched to the said lattice constant and having a back fielddopant concentration; epitaxially depositing a first barrier layer, uponsaid back field, of a semiconductor material that is lattice-matched tothe said lattice constant, and having a barrier dopant concentration anda barrier thickness grown at a barrier temperature. Additional stepsare: epitaxially depositing a quantum dot layer, comprising a highdensity of uniform self-assembled quantum dots with a low density ofdefects and having a shape and a size, upon previous barrier, using asemiconductor with a nominal composition for the quantum dots, that ishighly strained to the said lattice constant, and having a quantum dotdopant concentration, a quantum dot thickness, a quantum dot growthtemperature, a quantum dot growth rate, a quantum dot group Voverpressure or III-V ratio; epitaxially depositing a barrier layer,upon the previous quantum dot layer, after pausing the growth for agrowth interruption time, of a semiconductor material that islattice-matched to the said lattice constant, having a barrier dopantconcentration, a barrier thickness, a barrier growth rate, and a barriertemperature profile for the temperature of the substrate during theovergrowth of the quantum dots; and repeating the last 2 steps for anumber of periods, wherein said composition, said size, and said shapeof the quantum dots are controlled and can be changed throughout thestacking profile via the growth parameters. More steps include:epitaxially depositing top field layers, upon the previous barrierlayer, of semiconductor materials that are lattice-matched to the saidlattice constant and having a top field dopant concentration;epitaxially depositing a tunnel junction, upon said top field layers, ofhighly doped semiconductor materials that are lattice-matched to thesaid lattice constant and having a dopant concentration that is of thesame type as said top field dopant for the initial part of the tunneljunction and abruptly changed to the opposite type for the final part ofthe tunnel junction; providing a subcell lattice-matched to said latticeconstant upon the previous tunnel junction providing a tunnel junctionlattice-matched to said lattice constant upon the previous subcell andrepeating the last 2 steps to complete the number of subcells comprisedin the said multijunction solar cell. Finally, the method includes:providing a window upon the top subcell, providing an antireflectioncoating upon said window and providing a contact layer connected to saidtop subcell. In the method just described, the dopant concentration ofthe said buffer layers is between 1×10¹⁶ cm⁻³ and 1×10¹⁹ cm⁻³, saiddopant concentration of the said back field layers is between 1×10¹⁶cm⁻³ and 1×10¹⁹ cm⁻³, said barrier dopant concentration is between1×10¹³ cm⁻³ and 1×10¹⁷ cm⁻³, said quantum dot thickness is between 0.4nm to 5.0 nm, said quantum dot growth temperature is between 450° C. and540° C., said quantum dot growth rate is between 0.0001 nm/s and 0.2nm/s, said growth interruption time is between 0 s and 600 sec, saidbarrier temperature profile is either constant or varying between 450°C. and 650° C., said barrier growth rate is between 0.01 nm/s and 1nm/s, said barrier thickness is between 3 nm and 60 nm, said dopantconcentration of the said top field layers is between 1×10¹⁶ cm⁻³ and1×10¹⁹ cm⁻³. The lattice constant may be the lattice constant of GaAs,the alloy compositions of said buffer layers, said back field layers,said barrier material, and said top field layers, are between that ofAl_(0.3)Ga_(0.7)As and GaAs, and said nominal composition of the quantumdots is between that of In_(0.3)Ga_(0.7)As and InAs; the latticeconstant may be the lattice constant of GaAs, the alloy compositions ofsaid buffer layers, said back field layers, said barrier material, andsaid top field layers, are between that of Al_(0.9)Ga_(0.1) andAl_(0.1)Ga_(0.9)As, and said nominal composition of the quantum dots isbetween that of In_(0.3)Al_(0.7)As and InAs; or, the lattice constantmay be the lattice constant of GaAs, the alloy compositions of saidbuffer layers, said back field layers, said barrier material, and saidtop field layers, are that of GaAlInP alloys latticed-matched to GaAs,and said nominal composition of the quantum dots is InP.

A third embodiment of the invention provides high efficiency monolithicdual-junction photovoltaic solar cells. In another embodiment, aself-assembled quantum dot material is adapted to be used in adual-junction solar cell. In order to have a high efficiencyphotovoltaic dual-junction solar cell, a first subcell having aself-assembled quantum dot material similar to self-assembled quantumdot material 94, but with an absorption edge at 0.92 eV, is requiredtogether with a second subcell having a material absorbing photons withenergies greater than about 1.6 eV. The second subcell may also includea self-assembled quantum material similar to self-assembled quantum dotmaterial 94. Such a dual-junction solar cell would have balancedcurrents generated in each subcell. Furthermore, each subcell wouldabsorb about 31% of the total solar photon flux, as shown on plot 10 inFIG. 1 where bandgap 15 (1.6 eV) and bandgap 23 (0.92 eV) are depicted.

A dual-junction photovoltaic solar cell of the present invention isdepicted in FIG. 6. The dual-junction solar cell comprises a substrate144 upon which a first subcell 146 is pseudomorphically grown by epitaxyand comprises a first self-assembled In(Ga)As/GaAs quantum dot material,tailored to obtain an effective bandgap of about 0.92 eV. The details offirst subcell 146 are similar to the ones of second subcell 30 of FIG.2, disclosed in FIG. 4 and its corresponding description. A secondsubcell 148 is pseudomorphically grown by epitaxy on first subcell 146and may comprise a second self-assembled quantum dot material,preferably having AlInAs/AlGaAs quantum dots, and tailored to obtain aneffective bandgap of about 1.6 eV. In this embodiment, substrate 144 ispreferably a conductive GaAs substrate or a conductive Ge substrate,each of them having a similar lattice constant.

As discussed previously for other multijunction embodiments, the dopingof substrate 144 can be n-type or p-type. Whether substrate 144 isn-type with an n-p or an n-i-p unction grown on top or, p-type with ap-n or a p-i-n junction grown on top, is not fundamental to the presentinvention. For illustration purposes, this embodiment will use an n-typesubstrate with n-p or n-i-p junctions. Other possible combinations,which could include an undoped substrate and buried back contacts, areequally possible. In the final steps of the process, substrate 144 maybe metallized to form ohmic contact 150, as illustrated in FIG. 6.

A buffer and/or back field layer 152 can be grown on substrate 144 priorto growth of first subcell 146 in order to optimize various structural,electrical, or optical properties. First subcell 146 is electricallyconnected to second subcell 148 via tunnel junction 154. In thisembodiment tunnel junction 154 may comprise a highly doped InGaP, orAlGaAs n-p junction having a bandgap greater than about 1.7 eV.Alternatively, other similar AlInGaAsP alloys can be used. Secondsubcell 148 is essentially an n-p or an n-i-p junction pseudomorphicallygrown by epitaxy. As stated above, second subcell 148 may comprise aself-assembled AlInAs/AlGaAs quantum dot material, tailored to obtain aneffective bandgap of about 1.6 eV. Alternatively, second cell 148 may bemade of doped bulk GaInP or AlGaAs or other similar InAlGaAsP alloyshaving an alloy composition giving a bandgap of about 1.6 eV. Secondsubcell 148 may have window 156, antireflection coating 158, andelectrical contact 160, as is customary in the fabrication ofmultijunction solar cells.

The top part of FIG. 6 shows a graph 162 of the spectral intensity as afunction of wavelength for solar spectrum 50. Graph 162 also illustratesabsorption ranges 164 and 166 of solar spectrum 50 for first cell 146and second cell 148 respectively. It will be clear for someone skilledin the art, with the help of the description of FIG. 1, that such adual-junction photovoltage solar cell will have good current matchingbetween the subcells together with high conversion efficiencies.

For this embodiment, the structure of subcells having a self-assembledquantum dot material layer structure would be similar to that disclosedin FIG. 4 and its associated description. However, the growth conditionsare changed to obtain the desired optical, electrical and structuralproperties. In particular, the size and the composition of the quantumdot or the material adjacent to the quantum dots are modified to extendthe absorption of the self-assembled quantum dot material to longerwavelengths for first subcell 146, and to shorter wavelengths for secondsubcell 148. As discussed previously, there are many combinations ofparameters that can accomplish the desired goal.

For illustrations purposes of the present embodiment, the desiredabsorption characteristics of the self-assembled quantum dot material offirst cell 146 can be obtained by growing InAs on GaAs, the thickness ofInAs being comprised between 0.5 and 0.8 nm, preferably between 0.50 and0.58 nm. The preferred growth rate of InAs is comprised between 0.001and 3 nm/s, more preferably between 0.01 and 0.03 nm/s, with a growthpause following the InAs deposition (growth), the growth pausepreferably ranging from 0 to 300 seconds. The growth of the InAs quantumdot layer is followed by the over-growth of a barrier layer having athickness ranging from 6 nm to 50 nm, the barrier layer preferably beinga GaAs layer or an alloy having a similar bandgap. The growth sequenceof quantum dot layer and barrier layers is repeated a number of times asstated previously.

As mentioned above, a specific temperature cycling of substrate 144 maybe used to adjust the shape, composition, and uniformity of the quantumdots during the overgrowth of the quantum dot layer. In this case, thetemperature of substrate 144 is increased preferably above the InAsdisorption temperature and decreased back to its nominal value beforethe growth of the subsequent quantum dot layer. In the case where thedesired absorption edge is about 0.92 eV, the temperature cyclingperformed during the over-growth may occur when the thickness of thebarrier is comprised between 1 and 50 nm, preferably between 2 and 11nm, more preferably between 7.5 and 11 nm. Additionally, alloys ofslightly lower bandgap material than GaAs such as InGaAs with lowconcentration of indium, or graded bandgap material, can also be grownadjacent to the quantum dot layer in order to extend the absorption tolonger wavelengths.

A similar method is used to obtain the self-assembled quantum dotmaterial having the desired properties for second subcell 148; however,AlInAs quantum dots are used instead of InAs quantum dots and the GaAsbarrier material is replaced by an AlGaAs barrier material. The nominalpercentage of Al in the barrier can be between 0% and 100%, but ispreferably between 0% and 35%, in order to maintain a direct bandgapmaterial. More preferably, the Al percentage is comprised between 25%and 35%. The nominal percentage of In in the quantum dots can be between35% and 100%, but is preferably between 50% and 75%, and more preferablybetween 55% and 70%. The thickness of AlInAs used to form theself-assembled quantum dot layer is preferably comprised between 0.7 nmand 1.2 nm, and more preferably between 0.8 nm and 0.9 nm.

Alternatively, for second subcell 148, an equivalent structure can befabricated by using InP quantum dots instead of AlInAs quantum dots andGaInP barriers instead of AlGaAs barriers. Other alloys latticed-matchedto GaAs, such as GaAlInP or GaInPAs, can be used in the barriers.

The present invention can be applied to other embodiments and materials,for example, a dual-junction monolithic solar cell grown on an InPsubstrate. This dual-junction cell has a first subcell, which isessentially an n-p or n-i-p junction, preferably comprising InAs quantumdot material within InGaAs barriers lattice-matched to InP. Thedual-junction cell also has a second subcell, which is essentially ann-p junction preferably made of doped bulk AlInAs or of a similar alloylatticed-matched to InP such as AlInGaAs or GaInPAs. The first subcellof this embodiment has an extended absorption range compared to Ge sinceit absorbs photons of energies as small as about 0.65 eV. Thedisposition of the first and second subcells is the same as depicted inFIG. 6. The second subcell may be optional in some further embodiments.In an embodiment where the second subcell is optional, a lowerconversion efficiency might be obtained, but to the benefit of a simplermanufacturing and lower cost and/or higher radiation or defect hardness.

Another embodiment of interest uses a germanium substrate to form a highefficiency dual-junction monolithic photovoltaic solar cell. Here again,the disposition of the first and second subcells is as depicted in FIG.6. A first subcell, which is essentially an n-p or n-i-p junction, ismade of Ge pseudomorphically grown on the Ge substrate by epitaxy orother similar deposition, implantation, or interdiffusion techniques asdiscussed for subcell 110 of FIG. 5. The second subcell, which isessentially an n-p or n-i-p junction, preferably comprises InGaAsquantum dot material within AlGaAs barriers (or similar alloys such as,for example, AlGaP alloys) pseudomorphically grown by epitaxy with alattice constant close to that of Ge. Such a dual-junction cell using aGe substrate might not be as efficient as other possible embodimentsdiscussed herein but, will benefit from simpler manufacturing, higherflexibility from the wide combination of choices for the barrier layersand of the quantum dot layers, and from higher radiation and defectrobustness leading to higher end-of-life efficiencies.

Another embodiment particularly interesting uses a silicon substrate toform a high efficiency dual-junction monolithic photovoltaic solar cell.Here again, the disposition of the first and second cells is as depictedin FIG. 6. A first subcell, which is essentially an n-p or n-i-pjunction, preferably comprises Ge or SiGe quantum dot material and Sibarriers pseudomorphically grown on the Si substrate by epitaxy or othersimilar deposition techniques. Thin film methods or combinations ofmethods for growing semiconductor crystals can also be used. The secondsubcell, which is essentially an n-p or n-i-p junction, preferablycomprises InP quantum dot material within GaP barriers (or similaralloys such as such as AlGaP) pseudomorphically grown on Si by epitaxy.The dual-junction cell using a Si substrate can also be designed to useself-assembled quantum dot material in the second sub-cell grown on aconventional crystalline or poly-crystalline Si first cell in order toobtain improved efficiencies. The second subcell may be optional in somefurther embodiments. In an embodiment where the second subcell isoptional, a lower conversion efficiency might be obtained, but to thebenefit of a simpler manufacturing and lower cost.

As mentioned above, self-assembled quantum dot materials have been shownto produce devices that are orders of magnitude more radiation anddefect robust. The higher radiation and defect robustness is aconsequence of the combination of: (A) spatial confinement isolatingregions with defects, (B) favourable diffusion length damage coefficientdue to the nanostructures, (C) elimination of current limitingrestrictions in the subcells most affected by radiation, and (D) solarcell design without a current-limiting cell. This makes devices havingself-assembled quantum material particularly attractive for spaceapplications. The solar cells of the embodiments describe above wouldbenefit from radiation hardness in applications where the devices areexposed to radiations. For example, an optimized solar cell will have abeginning-of-life efficiency of about 40%. Assuming that this optimizedsolar cell has a radiation hardness improved by two orders of magnitudewith respect to conventional solar cells, the end-of-life efficiencywould be higher than 38% for an end-of-life defined as the equivalent toa total dose of 1×10¹⁵ cm⁻² of 1 MeV electron radiation.

It can be desirable to have embodiments of the present invention thatexploit and/or to optimize the defect hardness, even while partiallysacrificing some of the conversion efficiency. For example, adual-junction solar cell may include a Si substrate upon which ismetamorphically grown a plurality of GaAs/AlGaAs layers to make atransition buffer layer. On this transition buffer layer is epitaxiallygrown a first subcell comprising self-assembled In(Ga)As/GaAs quantumdot material, tailored to obtain an effective bandgap of about 0.92 eV.A second subcell, which could be optional in some embodiments, is grownby epitaxy on the first subcell and may comprise self-assembled quantumdot material preferably made of AlInAs/AlGaAs quantum dots, and adaptedto obtain an effective bandgap of about 1.6 eV. In this embodiment, asignificant concentration of defects will be presence due to the largelattice mismatch between Si and GaAs. However, the conversion efficiencycould be acceptable for some applications due to the defect hardness ofthe self-assembled quantum dot material.

In addition to defect hardness, other advantages of the invention may berealized. For example, a self-assembled quantum dot material can enhancethe conversion efficiency by recovering some of the photon energy thatis in excess of the semiconductor bandgap, which would otherwise be lostwhen photons having higher energies than the effective bandgap impingeon the multi-junction solar cell. As is well known, the energy in excessof the effective bandgap can generate phonons. Some of these phononswill be re-absorbed within the quantum dots and be used in thermionicemission processes by raising photocarriers from confined quantum dotstates to higher, unconfined, states, before being swept across thedepletion region. Hence the higher efficiency.

Other, similar schemes to optimize the conversion efficiency can beincorporated in embodiments of the invention. For example,self-assembled quantum dot materials can be doped with a controlledresidual doping to act as detectors in the longer infrared wavelengths.Detection processes would use intraband absorption simultaneously withthe interband transitions, and therefore increase the subcell currentsby using a larger fraction of the total solar photon flux. This is shownin FIG. 7 where a biased conduction band edge 200 is shown to form apotential well 202 at a quantum dot. Electronic quantized energy levels204, 206 and 208 are shown in potential well 202 together with thequasi-Fermi level 212. Electrons 300 are shown to populate levels 204and 206. Long wavelength infrared intraband absorption transitions(shown as 216 and 218), which are not normally present in solar cells,create additional photocarriers that drift in the device as shown byarrows 310. This type of absorption could be significant sinceapproximately 20% of the solar photon flux is in the energy range lyingbelow the Ge bandgap.

The proposed approach can also be extended to other embodiments andmaterial systems. For example semiconductor self-assembled quantum dotscontaining diluted nitrides such as InGaAsN or similar semiconductoralloys containing a small fraction of nitrogen, or In(Ga)N quantum dotswithin Ga(Al)N barriers, or using antimony-based material system such asInSb self-assembled quantum dots in Ga(Al)Sb barriers or similar alloys.Obviously the invention could also benefit applications other than solarenergy conversion, but also requiring the efficient conversion of abroadband source of photons into electrical signals.

A series of different embodiments of the invention have been presented.All the embodiments related to monolithic semiconductor photovoltaicsolar cells comprising at least one subcell having a self-assembledquantum dot material. Details on how to tailor the effective bandgap ofa subcell having a self-assembled quantum material were given. Thetailoring of the bandgap allows for solar cells having higher conversionefficiencies than prior art solar cells. Embodiments including two,three and four subcells were discussed.

The above-described embodiments of the present invention are intended tobe examples only. Alterations, modifications and variations may beeffected to the particular embodiments by those of skill in the artwithout departing from the scope of the invention, which is definedsolely by the claims appended hereto.

1. A monolithic, multijunction, semiconductor photovoltaic solar cell comprising: a plurality of subcells disposed in series, each subcell having formed therein a p-n junction or a p-i-n junction, the plurality of subcells having a quantum dot subcell, the quantum dot subcell being a subcell with strained epitaxially-grown semiconductor layers that include self-assembled quantum dots, the quantum dot subcell being pseudomorphically grown on another subcell.
 2. The solar cell of claim 1, further comprising: tunnel junctions formed between the subcells, the tunnel junctions for electrically connecting the subcells.
 3. The solar cell of claim 1, further comprising: an electrically conductive substrate upon which the plurality of subcells is formed.
 4. The solar cell of claim 3 wherein each of the subcells has an effective bandgap energy, the subcells being disposed in order of increasing effective bandgap energy, a subcell with a lowest effective bandgap energy being closest to the electrically conductive substrate.
 5. The solar cell of claim 1, wherein each of the subcells is for absorbing a substantially same fraction of solar photons.
 6. The solar cell of claim 4, wherein the plurality of subcells consists of three subcells, a first subcell having the lowest effective bandgap energy, a third subcell having a highest effective bandgap energy, and a second subcell disposed between the first subcell and the third subcell, the second subcell being the quantum dot subcell.
 7. The solar cell of claim 6 wherein: the electrically conductive substrate is a Ge or a GaAs substrate; the first subcell includes Ge; the strained epitaxially-grown semiconductor layers include strained InGaAs quantum dot layers intercalated with GaAs, AlGaAs, or GaPAs layers; and the third subcell includes GaInP, AlGaAs or AlGaInP.
 8. The solar cell of claim 7 wherein the first subcell is epitaxially grown on the electrically conductive substrate.
 9. The solar cell of claim 7 wherein the electrically conductive substrate is a Ge substrate and the first subcell is an interdiffused portion of the Ge substrate.
 10. The solar cell of claim 7 wherein: the electrically conductive substrate is n-doped; and an n-side of each of the p-n junction or p-i-n junction formed in each subcell is closer to the substrate than a respective p-side of the p-n junction or the p-i-n junction.
 11. The solar cell of claim 7 wherein: the electrically substrate is p-doped; and a p-side of the p-n junction of p-i-n junction formed in each subcell is closer to the substrate than a respective n-side of the p-n junction or p-i-n junction.
 12. The solar cell of claim 1 wherein: one of the subcells includes a Bragg reflector or a distributed Bragg reflector to modify an absorption characteristic of the solar cell.
 13. A monolithic, multijunction, semiconductor photovoltaic solar cell comprising: a plurality of subcells disposed in series, each subcell having formed therein a p-n junction or a p-i-n junction, the plurality of subcells having a quantum dot subcell, the quantum dot subcell being a subcell with strained epitaxially-grown semiconductor layers that include self-assembled quantum dots. 